The present invention relates to electroluminescent polymer compositions and processes for the preparation and use thereof in, for example, electroluminescent devices such as electroluminescent displays and electrochemical cells. More particularly, the invention relates to polymer compositions and to processes for making and using in, for example, high performance, that is, high stability and high luminosity electroluminescent displays and devices.
The present invention, in embodiments, is directed to processes for preparing soluble polymers, especially conjugated polymers, wherein at least one solubilizing side chain or substitutent which is present in the monomer imparts desirable solubility and processibility properties to the resulting polymer product.
Conjugated polymers are an important class of light emitting polymers for electroluminescent (EL) devices. There are two principal approaches to the fabrication of conjugated polymer thin films, namely, the precursor approach and side chain approach. The former relies on the preparation of a soluble precursor polymer which can be cast into thin films. The precursor polymer can then be converted to the final conjugated polymer films through solid-state thermo- or photo-conversion. Friend et al., disclosed EL devices based on poly(p-phenylene vinylene) (PPV) thin films derived from a sulfonium precursor route, reference U.S. Pat. No. 5,247,190. Hsieh et al., disclosed similar PPV EL devices using halogen precursor routes as disclosed in the aforementioned commonly owned U.S. Pat. No. 5,558,904. Son et al., reported PPV based EL devices using a xanthate precursor route in Science, 1995, 269, 376-380. There are several problems associated with the precursor approach. First, the precursor polymers derived from these precursor routes have unsatisfactory molecular structures because they are typically random copolymers containing 5-50% of converted segments. Second, expensive and potentially environmentally harmful photo- or thermo-conversion is required. Third, the presence of structural defects arising from incomplete thermoconversion and side reactions with volatile organic species generated during the thermal conversion. These aspects can lead to poor device properties and poor manufacture reproducibility.
The side chain approach of the present invention involves the formation of polymers by the polymerization of a monomer containing soluble substituents or side groups to afford a soluble conjugated polymer that can be cast into thin film directly without conversion. The polymerization of bis(halomethyl)benzenes in the presence of large excess base to provide PPV products was first reported by Gilch et al., in Journal of Polymer Science: Part A-1, 4:1337 (1966). In principle, the adaptation of the Gilch route to the polymerization of a 1,4-bis(halomethyl)benzene containing solubilizing groups should give a soluble PPV derivative. Unfortunately, this is not the situation in practice primarily because of polymer product precipitation during polymerization. The precipitation may be caused by the high molecular weight, semicrystallinity and/or cross-linking of the product. As a result, the desired soluble product has been obtained in very low yields (&lt;10%). The Gilch route is disadvantaged in that is lacks sufficient control over molecular weight of the polymer. Another problem of the Gilch route is the lack of control over polymer chain ends and polymer architecture. These problems to have been effectively solved by the use of non-polymerizing acidic additives as disclosed in the aforementioned copending U.S. Ser. No. 08/751,532, filed Nov. 13, 1996, now U.S. Pat. No. 5,817,430.
Conjugated polymers, such as poly(p-phenylene vinylenes), and electron acceptors, such as Buchminsterfullerenes C60, can be fabricated into heterojunction diodes for use in photodiodes and photovoltaic cells, reference for example, U.S. Pat. Nos. 5,454,880 and 5,331,183 both issued to Sariciffci, et. al. U.S. Pat. No. 5,401,827, issued Mar. 28, 1995, to Holmes et al., discloses various semiconductive conjugated copolymers containing arylene vinylene segments converted from sulfonium precursor polymers. The bandgap and refractive index of the conjugated copolymer were controlled by the degree of conversion. The patterning of such conjugated copolymers for device application was disclosed in U.S. Pat. No. 5,328,809, their application in luminescent devices was disclosed in U.S. Pat. No. 5,512,654, and their application in optical devices was disclosed in U.S. Pat. Nos. 5,425,125 and 5,523,555. U.S. Pat. No. 5,653,914, issued Aug. 5, 1997, to Holmes et al., discloses a processible polymer matrix such as polymethylmethacryate and a chromophoric component such as stilbene or distyrylbenzene for electroluminescent device application. The chromophoric components are blended into the polymer matrix or covalently attached thereto as a side group. U.S. Pat. Nos. 5,597,890 and 5,599,899, issued Jan. 28, 1997 and Feb. 4, 1997, to Jenekhe, disclose heterocyclic rigid rod and ladder polymers for electroluminescent device applications. Since the polymers were not soluble in organic solvents, they required dissolution in acid for film deposition, followed by water washing to remove residual acid. This acid processing scheme is complex, expensive, poor in reproducibility, and produces hazardous waste streams.